<ContentPage>
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition />
                <ColumnDefinition Width="auto" />
            </Grid.ColumnDefinitions>
            <CollectionView Grid.Column="0" 
                            SelectionMode="Single"
                            SelectedItem="{Binding SelectedItem, Mode=TwoWay}"
                            ItemsSource="{Binding ObservableCollectionTest}">
                <CollectionView.ItemsLayout>
                    <GridItemsLayout Orientation="Vertical" />
                </CollectionView.ItemsLayout>
                <CollectionView.ItemTemplate>
                    <DataTemplate>
                        <Grid BackgroundColor="Wheat">
                            <Label Text="{Binding}"></Label>
                        </Grid>
                    </DataTemplate>
                </CollectionView.ItemTemplate>
                
            </CollectionView>
            <Label Grid.Column="1"
                   Text="{Binding SelectedItem}" />
        </Grid>
    </ContentPage>

public static string SelectedItemPropertyName() { return nameof(SelectedItem); }
        private string _SelectedItem;
        public string SelectedItem
        {
            get
            {
                return _SelectedItem;
            }
            set
            {
                _SelectedItem = value;
                RaisePropertyChanged();
            }
        }
        public static string ObservableCollectionTestPropertyName() { return nameof(ObservableCollectionTest); }
        private ObservableCollection<string> _ObservableCollectionTest = new ObservableCollection<string>(new List<string>() { "Test1", "test2", "test3" } );
        public ObservableCollection<string> ObservableCollectionTest
        {
            get
            {
                return _ObservableCollectionTest;
            }
            set
            {
                _ObservableCollectionTest = value;
                RaisePropertyChanged();
            }
        }
SelectedItem = ObservableCollectionTest[1];